(function (global, factory) {
  typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
    typeof define === 'function' && define.amd ? define(factory) :
      (global.ResizeObserver = factory());
}(this, (function () {
  'use strict';

  /**
   * A collection of shims that provide minimal functionality of the ES6 collections.
   *
   * These implementations are not meant to be used outside of the ResizeObserver
   * modules as they cover only a limited range of use cases.
   */
  /* eslint-disable require-jsdoc, valid-jsdoc */
  var MapShim = (function () {
    if (typeof Map !== 'undefined') {
      return Map;
    }
    /**
     * Returns index in provided array that matches the specified key.
     *
     * @param {Array<Array>} arr
     * @param {*} key
     * @returns {number}
     */
    function getIndex(arr, key) {
      var result = -1;
      arr.some(function (entry, index) {
        if (entry[0] === key) {
          result = index;
          return true;
        }
        return false;
      });
      return result;
    }
    return /** @class */ (function () {
      function class_1() {
        this.__entries__ = [];
      }
      Object.defineProperty(class_1.prototype, "size", {
        /**
         * @returns {boolean}
         */
        get: function () {
          return this.__entries__.length;
        },
        enumerable: true,
        configurable: true
      });
      /**
       * @param {*} key
       * @returns {*}
       */
      class_1.prototype.get = function (key) {
        var index = getIndex(this.__entries__, key);
        var entry = this.__entries__[index];
        return entry && entry[1];
      };
      /**
       * @param {*} key
       * @param {*} value
       * @returns {void}
       */
      class_1.prototype.set = function (key, value) {
        var index = getIndex(this.__entries__, key);
        if (~index) {
          this.__entries__[index][1] = value;
        }
        else {
          this.__entries__.push([key, value]);
        }
      };
      /**
       * @param {*} key
       * @returns {void}
       */
      class_1.prototype.delete = function (key) {
        var entries = this.__entries__;
        var index = getIndex(entries, key);
        if (~index) {
          entries.splice(index, 1);
        }
      };
      /**
       * @param {*} key
       * @returns {void}
       */
      class_1.prototype.has = function (key) {
        return !!~getIndex(this.__entries__, key);
      };
      /**
       * @returns {void}
       */
      class_1.prototype.clear = function () {
        this.__entries__.splice(0);
      };
      /**
       * @param {Function} callback
       * @param {*} [ctx=null]
       * @returns {void}
       */
      class_1.prototype.forEach = function (callback, ctx) {
        if (ctx === void 0) { ctx = null; }
        for (var _i = 0, _a = this.__entries__; _i < _a.length; _i++) {
          var entry = _a[_i];
          callback.call(ctx, entry[1], entry[0]);
        }
      };
      return class_1;
    }());
  })();

  /**
   * Detects whether window and document objects are available in current environment.
   */
  var isBrowser = typeof window !== 'undefined' && typeof document !== 'undefined' && window.document === document;

  // Returns global object of a current environment.
  var global$1 = (function () {
    if (typeof global !== 'undefined' && global.Math === Math) {
      return global;
    }
    if (typeof self !== 'undefined' && self.Math === Math) {
      return self;
    }
    if (typeof window !== 'undefined' && window.Math === Math) {
      return window;
    }
    // eslint-disable-next-line no-new-func
    return Function('return this')();
  })();

  /**
   * A shim for the requestAnimationFrame which falls back to the setTimeout if
   * first one is not supported.
   *
   * @returns {number} Requests' identifier.
   */
  var requestAnimationFrame$1 = (function () {
    if (typeof requestAnimationFrame === 'function') {
      // It's required to use a bounded function because IE sometimes throws
      // an "Invalid calling object" error if rAF is invoked without the global
      // object on the left hand side.
      return requestAnimationFrame.bind(global$1);
    }
    return function (callback) { return setTimeout(function () { return callback(Date.now()); }, 1000 / 60); };
  })();

  // Defines minimum timeout before adding a trailing call.
  var trailingTimeout = 2;
  /**
   * Creates a wrapper function which ensures that provided callback will be
   * invoked only once during the specified delay period.
   *
   * @param {Function} callback - Function to be invoked after the delay period.
   * @param {number} delay - Delay after which to invoke callback.
   * @returns {Function}
   */
  function throttle(callback, delay) {
    var leadingCall = false, trailingCall = false, lastCallTime = 0;
    /**
     * Invokes the original callback function and schedules new invocation if
     * the "proxy" was called during current request.
     *
     * @returns {void}
     */
    function resolvePending() {
      if (leadingCall) {
        leadingCall = false;
        callback();
      }
      if (trailingCall) {
        proxy();
      }
    }
    /**
     * Callback invoked after the specified delay. It will further postpone
     * invocation of the original function delegating it to the
     * requestAnimationFrame.
     *
     * @returns {void}
     */
    function timeoutCallback() {
      requestAnimationFrame$1(resolvePending);
    }
    /**
     * Schedules invocation of the original function.
     *
     * @returns {void}
     */
    function proxy() {
      var timeStamp = Date.now();
      if (leadingCall) {
        // Reject immediately following calls.
        if (timeStamp - lastCallTime < trailingTimeout) {
          return;
        }
        // Schedule new call to be in invoked when the pending one is resolved.
        // This is important for "transitions" which never actually start
        // immediately so there is a chance that we might miss one if change
        // happens amids the pending invocation.
        trailingCall = true;
      }
      else {
        leadingCall = true;
        trailingCall = false;
        setTimeout(timeoutCallback, delay);
      }
      lastCallTime = timeStamp;
    }
    return proxy;
  }

  // Minimum delay before invoking the update of observers.
  var REFRESH_DELAY = 20;
  // A list of substrings of CSS properties used to find transition events that
  // might affect dimensions of observed elements.
  var transitionKeys = ['top', 'right', 'bottom', 'left', 'width', 'height', 'size', 'weight'];
  // Check if MutationObserver is available.
  var mutationObserverSupported = typeof MutationObserver !== 'undefined';
  /**
   * Singleton controller class which handles updates of ResizeObserver instances.
   */
  var ResizeObserverController = /** @class */ (function () {
    /**
     * Creates a new instance of ResizeObserverController.
     *
     * @private
     */
    function ResizeObserverController() {
      /**
       * Indicates whether DOM listeners have been added.
       *
       * @private {boolean}
       */
      this.connected_ = false;
      /**
       * Tells that controller has subscribed for Mutation Events.
       *
       * @private {boolean}
       */
      this.mutationEventsAdded_ = false;
      /**
       * Keeps reference to the instance of MutationObserver.
       *
       * @private {MutationObserver}
       */
      this.mutationsObserver_ = null;
      /**
       * A list of connected observers.
       *
       * @private {Array<ResizeObserverSPI>}
       */
      this.observers_ = [];
      this.onTransitionEnd_ = this.onTransitionEnd_.bind(this);
      this.refresh = throttle(this.refresh.bind(this), REFRESH_DELAY);
    }
    /**
     * Adds observer to observers list.
     *
     * @param {ResizeObserverSPI} observer - Observer to be added.
     * @returns {void}
     */
    ResizeObserverController.prototype.addObserver = function (observer) {
      if (!~this.observers_.indexOf(observer)) {
        this.observers_.push(observer);
      }
      // Add listeners if they haven't been added yet.
      if (!this.connected_) {
        this.connect_();
      }
    };
    /**
     * Removes observer from observers list.
     *
     * @param {ResizeObserverSPI} observer - Observer to be removed.
     * @returns {void}
     */
    ResizeObserverController.prototype.removeObserver = function (observer) {
      var observers = this.observers_;
      var index = observers.indexOf(observer);
      // Remove observer if it's present in registry.
      if (~index) {
        observers.splice(index, 1);
      }
      // Remove listeners if controller has no connected observers.
      if (!observers.length && this.connected_) {
        this.disconnect_();
      }
    };
    /**
     * Invokes the update of observers. It will continue running updates insofar
     * it detects changes.
     *
     * @returns {void}
     */
    ResizeObserverController.prototype.refresh = function () {
      var changesDetected = this.updateObservers_();
      // Continue running updates if changes have been detected as there might
      // be future ones caused by CSS transitions.
      if (changesDetected) {
        this.refresh();
      }
    };
    /**
     * Updates every observer from observers list and notifies them of queued
     * entries.
     *
     * @private
     * @returns {boolean} Returns "true" if any observer has detected changes in
     *      dimensions of it's elements.
     */
    ResizeObserverController.prototype.updateObservers_ = function () {
      // Collect observers that have active observations.
      var activeObservers = this.observers_.filter(function (observer) {
        return observer.gatherActive(), observer.hasActive();
      });
      // Deliver notifications in a separate cycle in order to avoid any
      // collisions between observers, e.g. when multiple instances of
      // ResizeObserver are tracking the same element and the callback of one
      // of them changes content dimensions of the observed target. Sometimes
      // this may result in notifications being blocked for the rest of observers.
      activeObservers.forEach(function (observer) { return observer.broadcastActive(); });
      return activeObservers.length > 0;
    };
    /**
     * Initializes DOM listeners.
     *
     * @private
     * @returns {void}
     */
    ResizeObserverController.prototype.connect_ = function () {
      // Do nothing if running in a non-browser environment or if listeners
      // have been already added.
      if (!isBrowser || this.connected_) {
        return;
      }
      // Subscription to the "Transitionend" event is used as a workaround for
      // delayed transitions. This way it's possible to capture at least the
      // final state of an element.
      document.addEventListener('transitionend', this.onTransitionEnd_);
      window.addEventListener('resize', this.refresh);
      if (mutationObserverSupported) {
        this.mutationsObserver_ = new MutationObserver(this.refresh);
        this.mutationsObserver_.observe(document, {
          attributes: true,
          childList: true,
          characterData: true,
          subtree: true
        });
      }
      else {
        document.addEventListener('DOMSubtreeModified', this.refresh);
        this.mutationEventsAdded_ = true;
      }
      this.connected_ = true;
    };
    /**
     * Removes DOM listeners.
     *
     * @private
     * @returns {void}
     */
    ResizeObserverController.prototype.disconnect_ = function () {
      // Do nothing if running in a non-browser environment or if listeners
      // have been already removed.
      if (!isBrowser || !this.connected_) {
        return;
      }
      document.removeEventListener('transitionend', this.onTransitionEnd_);
      window.removeEventListener('resize', this.refresh);
      if (this.mutationsObserver_) {
        this.mutationsObserver_.disconnect();
      }
      if (this.mutationEventsAdded_) {
        document.removeEventListener('DOMSubtreeModified', this.refresh);
      }
      this.mutationsObserver_ = null;
      this.mutationEventsAdded_ = false;
      this.connected_ = false;
    };
    /**
     * "Transitionend" event handler.
     *
     * @private
     * @param {TransitionEvent} event
     * @returns {void}
     */
    ResizeObserverController.prototype.onTransitionEnd_ = function (_a) {
      var _b = _a.propertyName, propertyName = _b === void 0 ? '' : _b;
      // Detect whether transition may affect dimensions of an element.
      var isReflowProperty = transitionKeys.some(function (key) {
        return !!~propertyName.indexOf(key);
      });
      if (isReflowProperty) {
        this.refresh();
      }
    };
    /**
     * Returns instance of the ResizeObserverController.
     *
     * @returns {ResizeObserverController}
     */
    ResizeObserverController.getInstance = function () {
      if (!this.instance_) {
        this.instance_ = new ResizeObserverController();
      }
      return this.instance_;
    };
    /**
     * Holds reference to the controller's instance.
     *
     * @private {ResizeObserverController}
     */
    ResizeObserverController.instance_ = null;
    return ResizeObserverController;
  }());

  /**
   * Defines non-writable/enumerable properties of the provided target object.
   *
   * @param {Object} target - Object for which to define properties.
   * @param {Object} props - Properties to be defined.
   * @returns {Object} Target object.
   */
  var defineConfigurable = (function (target, props) {
    for (var _i = 0, _a = Object.keys(props); _i < _a.length; _i++) {
      var key = _a[_i];
      Object.defineProperty(target, key, {
        value: props[key],
        enumerable: false,
        writable: false,
        configurable: true
      });
    }
    return target;
  });

  /**
   * Returns the global object associated with provided element.
   *
   * @param {Object} target
   * @returns {Object}
   */
  var getWindowOf = (function (target) {
    // Assume that the element is an instance of Node, which means that it
    // has the "ownerDocument" property from which we can retrieve a
    // corresponding global object.
    var ownerGlobal = target && target.ownerDocument && target.ownerDocument.defaultView;
    // Return the local global object if it's not possible extract one from
    // provided element.
    return ownerGlobal || global$1;
  });

  // Placeholder of an empty content rectangle.
  var emptyRect = createRectInit(0, 0, 0, 0);
  /**
   * Converts provided string to a number.
   *
   * @param {number|string} value
   * @returns {number}
   */
  function toFloat(value) {
    return parseFloat(value) || 0;
  }
  /**
   * Extracts borders size from provided styles.
   *
   * @param {CSSStyleDeclaration} styles
   * @param {...string} positions - Borders positions (top, right, ...)
   * @returns {number}
   */
  function getBordersSize(styles) {
    var positions = [];
    for (var _i = 1; _i < arguments.length; _i++) {
      positions[_i - 1] = arguments[_i];
    }
    return positions.reduce(function (size, position) {
      var value = styles['border-' + position + '-width'];
      return size + toFloat(value);
    }, 0);
  }
  /**
   * Extracts paddings sizes from provided styles.
   *
   * @param {CSSStyleDeclaration} styles
   * @returns {Object} Paddings box.
   */
  function getPaddings(styles) {
    var positions = ['top', 'right', 'bottom', 'left'];
    var paddings = {};
    for (var _i = 0, positions_1 = positions; _i < positions_1.length; _i++) {
      var position = positions_1[_i];
      var value = styles['padding-' + position];
      paddings[position] = toFloat(value);
    }
    return paddings;
  }
  /**
   * Calculates content rectangle of provided SVG element.
   *
   * @param {SVGGraphicsElement} target - Element content rectangle of which needs
   *      to be calculated.
   * @returns {DOMRectInit}
   */
  function getSVGContentRect(target) {
    var bbox = target.getBBox();
    return createRectInit(0, 0, bbox.width, bbox.height);
  }
  /**
   * Calculates content rectangle of provided HTMLElement.
   *
   * @param {HTMLElement} target - Element for which to calculate the content rectangle.
   * @returns {DOMRectInit}
   */
  function getHTMLElementContentRect(target) {
    // Client width & height properties can't be
    // used exclusively as they provide rounded values.
    var clientWidth = target.clientWidth, clientHeight = target.clientHeight;
    // By this condition we can catch all non-replaced inline, hidden and
    // detached elements. Though elements with width & height properties less
    // than 0.5 will be discarded as well.
    //
    // Without it we would need to implement separate methods for each of
    // those cases and it's not possible to perform a precise and performance
    // effective test for hidden elements. E.g. even jQuery's ':visible' filter
    // gives wrong results for elements with width & height less than 0.5.
    if (!clientWidth && !clientHeight) {
      return emptyRect;
    }
    var styles = getWindowOf(target).getComputedStyle(target);
    var paddings = getPaddings(styles);
    var horizPad = paddings.left + paddings.right;
    var vertPad = paddings.top + paddings.bottom;
    // Computed styles of width & height are being used because they are the
    // only dimensions available to JS that contain non-rounded values. It could
    // be possible to utilize the getBoundingClientRect if only it's data wasn't
    // affected by CSS transformations let alone paddings, borders and scroll bars.
    var width = toFloat(styles.width), height = toFloat(styles.height);
    // Width & height include paddings and borders when the 'border-box' box
    // model is applied (except for IE).
    if (styles.boxSizing === 'border-box') {
      // Following conditions are required to handle Internet Explorer which
      // doesn't include paddings and borders to computed CSS dimensions.
      //
      // We can say that if CSS dimensions + paddings are equal to the "client"
      // properties then it's either IE, and thus we don't need to subtract
      // anything, or an element merely doesn't have paddings/borders styles.
      if (Math.round(width + horizPad) !== clientWidth) {
        width -= getBordersSize(styles, 'left', 'right') + horizPad;
      }
      if (Math.round(height + vertPad) !== clientHeight) {
        height -= getBordersSize(styles, 'top', 'bottom') + vertPad;
      }
    }
    // Following steps can't be applied to the document's root element as its
    // client[Width/Height] properties represent viewport area of the window.
    // Besides, it's as well not necessary as the <html> itself neither has
    // rendered scroll bars nor it can be clipped.
    if (!isDocumentElement(target)) {
      // In some browsers (only in Firefox, actually) CSS width & height
      // include scroll bars size which can be removed at this step as scroll
      // bars are the only difference between rounded dimensions + paddings
      // and "client" properties, though that is not always true in Chrome.
      var vertScrollbar = Math.round(width + horizPad) - clientWidth;
      var horizScrollbar = Math.round(height + vertPad) - clientHeight;
      // Chrome has a rather weird rounding of "client" properties.
      // E.g. for an element with content width of 314.2px it sometimes gives
      // the client width of 315px and for the width of 314.7px it may give
      // 314px. And it doesn't happen all the time. So just ignore this delta
      // as a non-relevant.
      if (Math.abs(vertScrollbar) !== 1) {
        width -= vertScrollbar;
      }
      if (Math.abs(horizScrollbar) !== 1) {
        height -= horizScrollbar;
      }
    }
    return createRectInit(paddings.left, paddings.top, width, height);
  }
  /**
   * Checks whether provided element is an instance of the SVGGraphicsElement.
   *
   * @param {Element} target - Element to be checked.
   * @returns {boolean}
   */
  var isSVGGraphicsElement = (function () {
    // Some browsers, namely IE and Edge, don't have the SVGGraphicsElement
    // interface.
    if (typeof SVGGraphicsElement !== 'undefined') {
      return function (target) { return target instanceof getWindowOf(target).SVGGraphicsElement; };
    }
    // If it's so, then check that element is at least an instance of the
    // SVGElement and that it has the "getBBox" method.
    // eslint-disable-next-line no-extra-parens
    return function (target) {
      return (target instanceof getWindowOf(target).SVGElement &&
        typeof target.getBBox === 'function');
    };
  })();
  /**
   * Checks whether provided element is a document element (<html>).
   *
   * @param {Element} target - Element to be checked.
   * @returns {boolean}
   */
  function isDocumentElement(target) {
    return target === getWindowOf(target).document.documentElement;
  }
  /**
   * Calculates an appropriate content rectangle for provided html or svg element.
   *
   * @param {Element} target - Element content rectangle of which needs to be calculated.
   * @returns {DOMRectInit}
   */
  function getContentRect(target) {
    if (!isBrowser) {
      return emptyRect;
    }
    if (isSVGGraphicsElement(target)) {
      return getSVGContentRect(target);
    }
    return getHTMLElementContentRect(target);
  }
  /**
   * Creates rectangle with an interface of the DOMRectReadOnly.
   * Spec: https://drafts.fxtf.org/geometry/#domrectreadonly
   *
   * @param {DOMRectInit} rectInit - Object with rectangle's x/y coordinates and dimensions.
   * @returns {DOMRectReadOnly}
   */
  function createReadOnlyRect(_a) {
    var x = _a.x, y = _a.y, width = _a.width, height = _a.height;
    // If DOMRectReadOnly is available use it as a prototype for the rectangle.
    var Constr = typeof DOMRectReadOnly !== 'undefined' ? DOMRectReadOnly : Object;
    var rect = Object.create(Constr.prototype);
    // Rectangle's properties are not writable and non-enumerable.
    defineConfigurable(rect, {
      x: x, y: y, width: width, height: height,
      top: y,
      right: x + width,
      bottom: height + y,
      left: x
    });
    return rect;
  }
  /**
   * Creates DOMRectInit object based on the provided dimensions and the x/y coordinates.
   * Spec: https://drafts.fxtf.org/geometry/#dictdef-domrectinit
   *
   * @param {number} x - X coordinate.
   * @param {number} y - Y coordinate.
   * @param {number} width - Rectangle's width.
   * @param {number} height - Rectangle's height.
   * @returns {DOMRectInit}
   */
  function createRectInit(x, y, width, height) {
    return { x: x, y: y, width: width, height: height };
  }

  /**
   * Class that is responsible for computations of the content rectangle of
   * provided DOM element and for keeping track of it's changes.
   */
  var ResizeObservation = /** @class */ (function () {
    /**
     * Creates an instance of ResizeObservation.
     *
     * @param {Element} target - Element to be observed.
     */
    function ResizeObservation(target) {
      /**
       * Broadcasted width of content rectangle.
       *
       * @type {number}
       */
      this.broadcastWidth = 0;
      /**
       * Broadcasted height of content rectangle.
       *
       * @type {number}
       */
      this.broadcastHeight = 0;
      /**
       * Reference to the last observed content rectangle.
       *
       * @private {DOMRectInit}
       */
      this.contentRect_ = createRectInit(0, 0, 0, 0);
      this.target = target;
    }
    /**
     * Updates content rectangle and tells whether it's width or height properties
     * have changed since the last broadcast.
     *
     * @returns {boolean}
     */
    ResizeObservation.prototype.isActive = function () {
      var rect = getContentRect(this.target);
      this.contentRect_ = rect;
      return (rect.width !== this.broadcastWidth ||
        rect.height !== this.broadcastHeight);
    };
    /**
     * Updates 'broadcastWidth' and 'broadcastHeight' properties with a data
     * from the corresponding properties of the last observed content rectangle.
     *
     * @returns {DOMRectInit} Last observed content rectangle.
     */
    ResizeObservation.prototype.broadcastRect = function () {
      var rect = this.contentRect_;
      this.broadcastWidth = rect.width;
      this.broadcastHeight = rect.height;
      return rect;
    };
    return ResizeObservation;
  }());

  var ResizeObserverEntry = /** @class */ (function () {
    /**
     * Creates an instance of ResizeObserverEntry.
     *
     * @param {Element} target - Element that is being observed.
     * @param {DOMRectInit} rectInit - Data of the element's content rectangle.
     */
    function ResizeObserverEntry(target, rectInit) {
      var contentRect = createReadOnlyRect(rectInit);
      // According to the specification following properties are not writable
      // and are also not enumerable in the native implementation.
      //
      // Property accessors are not being used as they'd require to define a
      // private WeakMap storage which may cause memory leaks in browsers that
      // don't support this type of collections.
      defineConfigurable(this, { target: target, contentRect: contentRect });
    }
    return ResizeObserverEntry;
  }());

  var ResizeObserverSPI = /** @class */ (function () {
    /**
     * Creates a new instance of ResizeObserver.
     *
     * @param {ResizeObserverCallback} callback - Callback function that is invoked
     *      when one of the observed elements changes it's content dimensions.
     * @param {ResizeObserverController} controller - Controller instance which
     *      is responsible for the updates of observer.
     * @param {ResizeObserver} callbackCtx - Reference to the public
     *      ResizeObserver instance which will be passed to callback function.
     */
    function ResizeObserverSPI(callback, controller, callbackCtx) {
      /**
       * Collection of resize observations that have detected changes in dimensions
       * of elements.
       *
       * @private {Array<ResizeObservation>}
       */
      this.activeObservations_ = [];
      /**
       * Registry of the ResizeObservation instances.
       *
       * @private {Map<Element, ResizeObservation>}
       */
      this.observations_ = new MapShim();
      if (typeof callback !== 'function') {
        throw new TypeError('The callback provided as parameter 1 is not a function.');
      }
      this.callback_ = callback;
      this.controller_ = controller;
      this.callbackCtx_ = callbackCtx;
    }
    /**
     * Starts observing provided element.
     *
     * @param {Element} target - Element to be observed.
     * @returns {void}
     */
    ResizeObserverSPI.prototype.observe = function (target) {
      if (!arguments.length) {
        throw new TypeError('1 argument required, but only 0 present.');
      }
      // Do nothing if current environment doesn't have the Element interface.
      if (typeof Element === 'undefined' || !(Element instanceof Object)) {
        return;
      }
      if (!(target instanceof getWindowOf(target).Element)) {
        throw new TypeError('parameter 1 is not of type "Element".');
      }
      var observations = this.observations_;
      // Do nothing if element is already being observed.
      if (observations.has(target)) {
        return;
      }
      observations.set(target, new ResizeObservation(target));
      this.controller_.addObserver(this);
      // Force the update of observations.
      this.controller_.refresh();
    };
    /**
     * Stops observing provided element.
     *
     * @param {Element} target - Element to stop observing.
     * @returns {void}
     */
    ResizeObserverSPI.prototype.unobserve = function (target) {
      if (!arguments.length) {
        throw new TypeError('1 argument required, but only 0 present.');
      }
      // Do nothing if current environment doesn't have the Element interface.
      if (typeof Element === 'undefined' || !(Element instanceof Object)) {
        return;
      }
      if (!(target instanceof getWindowOf(target).Element)) {
        throw new TypeError('parameter 1 is not of type "Element".');
      }
      var observations = this.observations_;
      // Do nothing if element is not being observed.
      if (!observations.has(target)) {
        return;
      }
      observations.delete(target);
      if (!observations.size) {
        this.controller_.removeObserver(this);
      }
    };
    /**
     * Stops observing all elements.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.disconnect = function () {
      this.clearActive();
      this.observations_.clear();
      this.controller_.removeObserver(this);
    };
    /**
     * Collects observation instances the associated element of which has changed
     * it's content rectangle.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.gatherActive = function () {
      var _this = this;
      this.clearActive();
      this.observations_.forEach(function (observation) {
        if (observation.isActive()) {
          _this.activeObservations_.push(observation);
        }
      });
    };
    /**
     * Invokes initial callback function with a list of ResizeObserverEntry
     * instances collected from active resize observations.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.broadcastActive = function () {
      // Do nothing if observer doesn't have active observations.
      if (!this.hasActive()) {
        return;
      }
      var ctx = this.callbackCtx_;
      // Create ResizeObserverEntry instance for every active observation.
      var entries = this.activeObservations_.map(function (observation) {
        return new ResizeObserverEntry(observation.target, observation.broadcastRect());
      });
      this.callback_.call(ctx, entries, ctx);
      this.clearActive();
    };
    /**
     * Clears the collection of active observations.
     *
     * @returns {void}
     */
    ResizeObserverSPI.prototype.clearActive = function () {
      this.activeObservations_.splice(0);
    };
    /**
     * Tells whether observer has active observations.
     *
     * @returns {boolean}
     */
    ResizeObserverSPI.prototype.hasActive = function () {
      return this.activeObservations_.length > 0;
    };
    return ResizeObserverSPI;
  }());

  // Registry of internal observers. If WeakMap is not available use current shim
  // for the Map collection as it has all required methods and because WeakMap
  // can't be fully polyfilled anyway.
  var observers = typeof WeakMap !== 'undefined' ? new WeakMap() : new MapShim();
  /**
   * ResizeObserver API. Encapsulates the ResizeObserver SPI implementation
   * exposing only those methods and properties that are defined in the spec.
   */
  var ResizeObserver = /** @class */ (function () {
    /**
     * Creates a new instance of ResizeObserver.
     *
     * @param {ResizeObserverCallback} callback - Callback that is invoked when
     *      dimensions of the observed elements change.
     */
    function ResizeObserver(callback) {
      if (!(this instanceof ResizeObserver)) {
        throw new TypeError('Cannot call a class as a function.');
      }
      if (!arguments.length) {
        throw new TypeError('1 argument required, but only 0 present.');
      }
      var controller = ResizeObserverController.getInstance();
      var observer = new ResizeObserverSPI(callback, controller, this);
      observers.set(this, observer);
    }
    return ResizeObserver;
  }());
  // Expose public methods of ResizeObserver.
  [
    'observe',
    'unobserve',
    'disconnect'
  ].forEach(function (method) {
    ResizeObserver.prototype[method] = function () {
      var _a;
      return (_a = observers.get(this))[method].apply(_a, arguments);
    };
  });

  var index = (function () {
    // Export existing implementation if available.
    if (typeof global$1.ResizeObserver !== 'undefined') {
      return global$1.ResizeObserver;
    }
    return ResizeObserver;
  })();

  return index;

})));